Most metazoans develop into adults of characteristic size and shape, properties that are determined by the balance of cellular proliferation, apoptosis, and growth. While a great deal is known about the individual pathways that regulate these three processes, much less is understood about how they are coordinately regulated. A recently identified family of factors, the NHL domain proteins, appears to play such a role in a wide variety of organisms. This idea is supported primarily by genetic studies of C. elegans nc/-i and lin-41 and the three NHL domain genes in Drosophila-- brain tumor, mei-P26, and dappled. The molecular mechanism by which the encoded proteins act has not been clear. We have made two critical observations which suggest that the Drosophila NHL proteins regulate post-transcriptional gene expression, thereby controlling organismal size. First, mis-expression of Brain Tumor and Mei-P26 in the imaginal discs causes dramatic non-autonomous reductions in the size of the corresponding adult structures. Second, substitutions in the NHL domain of Brain Tumor abrogate both translational control of hunchback mRNA in the early embryo and the size defects observed upon mis-expression in imaginal discs. In this proposal, we will investigate the cellular mechanisms by which NHL domain proteins regulate size, taking advantage of the excellent tools that have been developed for investigation of proliferation, apoptosis, and growth of imaginal disc cells. We will determine the cellular response to: (1) gain-of-NHL protein function, (2) loss-of-NHL protein function, and (3) simultaneous loss-of-function of two NHL domain proteins (to address possible redundancy). To identify the molecular mechanisms by which they function, we will: (4) isolate factors that interact with wild type NHL domains but not mutant derivatives that are inactive in vivo. Taken together, these experiments will establish the basis for understanding the cellular and molecular processes controlled by NHL domain proteins in Drosophila, and by extension, other organisms.